Break circular dependency: init -> * -> init by extracting shutdown.h

Most includers just wanted to react to pending shutdown.

This isolates access to `fRequestShutdown` and limits access to the shutdown
api functions, including the new `AbortShutdown` for setting it to `false`.

Note I originally called `AbortShutdown` `CancelShutdown` but that name was
already taken by winuser.h
https://travis-ci.org/bitcoin/bitcoin/jobs/386913329

This change also triggered a build error in bench. Fixing it required moving LIBBITCOIN_SERVER after LIBBITCOIN_WALLET in bench_bench_bitcoin_LDADD To make
server definitions in src/net.cpp available to wallet methods in
src/wallet/wallet.cpp. Specifically, solving:

  libbitcoin_wallet.a(libbitcoin_wallet_a-wallet.o): In function `CWalletTx::RelayWalletTransaction(CConnman*)':
  wallet.cpp:(.text+0x3f0e): undefined reference to `CConnman::NodeFullyConnected(CNode const*)'
  collect2: error: ld returned 1 exit status

https://travis-ci.org/bitcoin/bitcoin/jobs/392133581

Need for remaining init.h includes confirmed via a thorough search with a more
specific regex:
  \bInterrupt\(\)|\bShutdown\(\)|\bInitLogging\(\)|\bInitParameterInteraction\(\)|\bAppInitBasicSetup\(\)|\bAppInitParameterInteraction\(\)|\bAppInitSanityChecks\(\)|\bAppInitLockDataDirectory\(\)|\bAppInitMain\(\)|\bSetupServerArgs\(\)|\bLicenseInfo\(\)|g_wallet_init_interface|init.h

src/Makefile.bench.include

@@ -34,8 +34,8 @@ nodist_bench_bench_bitcoin_SOURCES = $(GENERATED_BENCH_FILES)
 bench_bench_bitcoin_CPPFLAGS = $(AM_CPPFLAGS) $(BITCOIN_INCLUDES) $(EVENT_CLFAGS) $(EVENT_PTHREADS_CFLAGS) -I$(builddir)/bench/
 bench_bench_bitcoin_CXXFLAGS = $(AM_CXXFLAGS) $(PIE_FLAGS)
 bench_bench_bitcoin_LDADD = \
-  $(LIBBITCOIN_SERVER) \
   $(LIBBITCOIN_WALLET) \
+  $(LIBBITCOIN_SERVER) \
   $(LIBBITCOIN_COMMON) \
   $(LIBBITCOIN_UTIL) \
   $(LIBBITCOIN_CONSENSUS) \